Augmented Raman analysis of a gas mixture

ABSTRACT

The present disclosure includes discloses a method for analyzing a multi-component gas sample using spectroscopy in combination with the measurement of extrinsic or intrinsic properties of the gas sample. The results of the spectroscopic analysis and the measurement are combined to quantify a gas component unseen by the spectroscopic analysis.

CROSS-REFERENCE TO RELATED APPLICATION

The present application is a divisional application of U.S. patentapplication Ser. No. 15/830,094, filed on Dec. 4, 2017, the entirecontents of which are incorporated herein by reference.

TECHNICAL FIELD

The present disclosure relates generally to analyzing gas mixtures usingRaman spectroscopy.

BACKGROUND OF THE INVENTION

Composition analysis of a gas sample by Raman spectroscopy allows themeasurement of the abundance of the chemical species within the gassample for those chemical species composed of at least two atoms.Species composed of just one atom, such as the noble gases helium andargon, and ionic-bonded substances like salts, are invisible to Ramanspectroscopy. In practice, such Raman-invisible species are present insome chemical mixtures and often need to be quantified.

Absorption spectroscopy such as near infrared spectroscopy and infraredspectroscopy suffer similar limitations as the Raman analysis. That is,substances such as homonuclear diatomics (O₂, N₂, etc.) and ionic-bondedsalts are invisible to the absorption spectroscopy, but neverthelessthese substances are present in some chemical mixtures and often need tobe quantified.

In sample streams containing a component invisible to Raman orabsorption spectroscopy, it is often necessary to supplement thespectroscopy measurements to quantify invisible species and obtain fullcomposition analysis. For example, in the synthesis loop in a fertilizerplant argon is present along with other gases. Though Raman spectroscopycan characterize most of the gases in a sample, the detection of argonrequires another technology such as gas chromatography or massspectrometry. These methods are relatively expensive and not well suitedto in situ detection within a pipeline or a reactor.

Less costly analysis methods exist which can quantify theRaman-invisible species, but such methods work well for mixtures withonly a few components, ideally mixtures containing only two gases, i.e.,binary mixtures. But in many industrial applications many gases will bepresent in a sample, and a direct application of methods for solvingbinary mixtures must be modified, and assumptions must be made regardingwhich components are changing and which are stable. Accordingly, thereremains a need for further contributions in this area of technology toenable compositional analysis of mixtures that includespectroscopic-invisible species.

SUMMARY OF THE INVENTION

The present disclosure discloses a method for analyzing the compositionof a multi-component matter sample including determining a firstcomposition of a multi-component matter sample using a spectroscopicdevice; calculating a relative composition matrix containing anormalized molar amount of each component of the matter sample, based onthe first composition; calculating a value of a first secondary propertyof the matter sample using the relative composition matrix; measuring avalue of the first secondary property of the matter sample with a sensorembodied to measure the value of the first secondary property; anddetermining whether a first difference between the calculated value ofthe first secondary property and the measured value of the firstsecondary property exceeds a first threshold, wherein upon determiningthat the first difference exceeds the first threshold: attributing thefirst difference to a component invisible to the spectroscopic device;calculating a first amount of the spectroscopic-invisible componentusing the first difference; adding the first amount of thespectroscopic-invisible component to the relative composition matrix;and adjusting the normalized molar amount of each component in therelative composition matrix to account for the first amount of thespectroscopic-invisible component.

The method may include using a Raman spectroscopic device, andspectroscopic-invisible would be defined as invisible to Ramanspectroscopy. Alternately, the method may include using a near infraredabsorption spectroscopic device, and spectroscopic-invisible would bedefined as invisible to near infrared absorption spectroscopy.Alternately, the method may include using an infrared absorptionspectroscopic device, and spectroscopic-invisible would be defined asinvisible to infrared absorption spectroscopy.

The method may be employed for matter in a gaseous phase or in acondensed phase.

The first secondary property may be one of the following: thermalconductivity, electrical conductivity, viscosity, pH, density,turbidity, and color-changing chemical reaction.

In an embodiment, the method may include calculating a value of a secondsecondary property of the matter sample using the relative compositionmatrix, the second secondary property being different from the firstsecondary property; measuring a value of the second secondary propertyof the matter sample with a sensor embodied to measure the value of thesecond secondary property; and determining whether a second differencebetween the calculated value of the second secondary property and themeasured value of the second secondary property exceeds a secondthreshold, wherein upon determining that the second difference exceedsthe second threshold: attributing the second difference to thespectroscopic-invisible component present in the sample; calculating asecond amount of the spectroscopic-invisible component using the seconddifference; calculating a third amount of the spectroscopic-invisiblecomponent using the weighted average of the first amount of thespectroscopic-invisible component and the second amount of thespectroscopic-invisible component; replacing the first amount of thespectroscopic-invisible component in the relative composition matrixwith the third amount of the spectroscopic-invisible component; andadjusting the normalized molar amounts of each component in the relativecomposition matrix to account for the third amount of thespectroscopic-invisible component.

In another embodiment, a method for analyzing the composition of amulti-component gas sample includes: determining a first composition ofa multi-component gas sample using a spectroscopic device; calculating arelative composition matrix containing a normalized molar amount of eachcomponent of the multi-component gas sample, based on the firstcomposition; calculating an average molar mass of the relativecomposition matrix using a molar mass and the normalized molar amount ofeach component of the relative composition matrix; measuring atemperature, a pressure, and a density of the gas sample and calculatingan inverse compressibility factor of the gas sample therefrom;calculating a molar amount of a spectroscopic-invisible gas using theaverage molar mass, the inverse compressibility factor, and a molar massof the spectroscopic-invisible gas; adding the molar amount of thespectroscopic-invisible gas to the relative composition matrix; andadjusting the normalized molar amount of each component in the relativecomposition matrix to account for the molar amount of thespectroscopic-invisible gas.

The method may include using a Raman spectroscopic device, andspectroscopic-invisible would be defined as invisible to Ramanspectroscopy. Alternately, the method may include using a near infraredabsorption spectroscopic device, and spectroscopic-invisible would bedefined as invisible to near infrared absorption spectroscopy.Alternately, the method may include using an infrared absorptionspectroscopic device, and spectroscopic-invisible would be defined asinvisible to infrared absorption spectroscopy.

In another embodiment, a method for analyzing the composition of amulti-component gas sample includes determining a first composition of amulti-component gas sample using a spectroscopic device; calculating arelative composition matrix containing a normalized molar amount of eachcomponent of the multi-component gas sample, based on the firstcomposition; measuring a temperature, a pressure, and a density of thegas sample and calculating a first compressibility factor of the gassample therefrom; estimating a molar amount of a spectroscopic-invisiblegas; adding the estimated molar amount of the spectroscopic-invisiblegas to the relative composition matrix; adjusting the normalized molaramount of the other components in the relative composition matrix toaccount for the estimated molar amount of the spectroscopic-invisiblegas; calculating a second compressibility factor of the gas sample usingthe relative composition matrix; determining whether a differencebetween the first compressibility factor and the second compressibilityfactor exceeds a threshold, wherein upon determining that the differenceexceeds the threshold: adjusting the estimated molar amount of thespectroscopic-invisible gas in the relative composition matrix such thatthe difference between the first compressibility factor and the secondcompressibility factor is reduced; repeating the adjusting thenormalized molar amount of each component in the relative compositionmatrix, the calculating a second compressibility factor, and thedetermining whether a difference between the first compressibilityfactor and the second compressibility factor exceeds a threshold untilthe difference does not exceed the threshold.

The method may include using a Raman spectroscopic device andspectroscopic-invisible would be defined as invisible to Ramanspectroscopy. Alternately the method may include using a nearinfrared/infrared absorption spectroscopic device andspectroscopic-invisible would be defined as invisible to nearinfrared/infrared absorption spectroscopy.

BRIEF DESCRIPTION OF THE DRAWINGS

The described embodiments and other features, advantages, anddisclosures contained herein, and the matter of attaining them, willbecome apparent and the present disclosure will be better understood byreference to the following description of various embodiments of thepresent disclosure taken in conjunction with the accompanying drawings,wherein:

FIG. 1 shows the flow diagram of an embodiment of the disclosed method;

FIG. 2 shows an exemplary calculation of the mixture composition usingan ideal gas approximation; and

FIG. 3 shows an exemplary calculation of the mixture composition using anon-linear iterative method.

DETAILED DESCRIPTION

The present disclosure discloses systems and methods for determining thequantities of constituent species of a gas or condensed-phase mixture,including spectroscopic-invisible species. According to at least oneaspect of the present disclosure, the methods include supplementing afirst analysis of a multi-component sample with a second analysis ofthat sample. The first analysis may not quantify all of the componentsin the sample; therefore, a second analysis using tools, detectors, andsensors different from those used in the first analysis is performed,and the results of the two analyses are combined to determine the fullcomposition of the multi-component sample.

For the purposes of promoting an understanding of the principles of thepresent disclosure, reference will be made to the embodimentsillustrated in the drawings, and specific language will be used todescribe the same. It will nevertheless be understood that no limitationof the scope of this disclosure is thereby intended.

In at least one embodiment of the present disclosure, the first analysisis performed on a multi-component gas sample using Raman spectroscopy.The first analysis (i.e., the Raman analysis) determines the type andamount of each chemical species present in the gas sample that has aRaman signature. Those substances present that do not have a Ramansignature, e.g., substances that do not form molecules or that areionically bonded, are not detected by Raman spectroscopy because they donot wavelength-shift the light they scatter. Therefore, a secondanalysis, one which does not use Raman spectroscopy, is performed tomeasure the presence of the Raman-invisible components.

The second analysis includes the calculation of one or more propertiesof the gas sample that can also be measured. Accordingly, the secondanalysis includes the measurement of one or more properties of the gassample that can also be calculated when the constituents of the gassample are known. Therefore, the choice of a property of the gas sampleto be used in the second analysis, herein referred to as a secondaryproperty, requires that the secondary property can be both calculatedand measured.

The term secondary property in this disclosure refers to an extrinsic orintrinsic property of the mixture or of a component of the mixture. Suchextrinsic or intrinsic properties include density, thermal conductivity,electrical conductivity, viscosity, turbidity, pH, and color-changingchemical reaction. The primary property of the gas mixture or of acomponent of the gas mixture as used in this disclosure is the molaramount of the substance.

The disclosed methods are most applicable when it is known, or at leastexpected, that a particular spectroscopic-invisible component is presentin the gas or the condensed-phase mixture. For example, argon gas isoften present in the synthesis loop in an ammonia production process. Asanother example, helium is often present in natural gas extracted fromthe earth. Knowledge of the type of the spectroscopic-invisiblecomponent enables the calculation of the secondary property chosen forthe second analysis.

A method 100 according to at least one embodiment of the presentdisclosure is shown in FIG. 1 . The method 100 may be performed on amulti-component gas sample contained in a sample chamber.

The method 100 includes a step 110 of analyzing a multi-component gassample using a Raman spectrometer device. The Raman spectrometergenerates and captures a spectrum of Raman-scattered light.

The method 100 may include a step 115 of retrieving from a databasecalibration data from a previously performed calibration of the Ramandevice and adjusting the Raman spectrum using the retrieved calibrationdata.

The method 100 includes a step 120 of calculating the relativecomposition of the gas sample from the adjusted Raman spectrum andproducing a matrix of the components of the gas sample. The step 120 mayinclude using the calibration data retrieved in step 115 to improve theaccuracy of the analysis. The step 120 includes normalizing thecomponents of the matrix by expressing each Raman-detected component asa ratio of the amount of that component to the sum of the amounts of allRaman-detected components. That is, each component is expressed as apercentage of the whole sample. The result of step 120 is a relativecomposition matrix containing the amounts of each Raman-detectedcomponent in the gas sample.

The method 100 includes a step 130 of calculating the value of at leastone secondary property of the gas sample using the relative compositionmatrix as determined in step 120. The secondary property of the gassample may be an intrinsic property such as thermal conductivity,electrical conductivity, viscosity, and pH, among others. The secondaryproperty of the gas sample may be an extrinsic property such as densityand turbidity, among others. The step 130 may include calculating thevalue of more than one secondary property.

The method 100 includes a step 140 of measuring the value of thesecondary property which was calculated in step 130. The measurement ofthe secondary property (or properties) is performed on the gas sample inthe sample chamber using a device embodied to measure that secondaryproperty.

The choice of the secondary property may be partially determined by theavailability of a capable measuring device or sensor having sufficientresolution to measure the secondary property of the Raman-invisiblecomponent. For example, if the chosen secondary property of the gassample were thermal conductivity, the resolution of the thermalconductivity measuring device must be greater than the contribution tothermal conductivity that the Raman-invisible gas provides. In certainembodiments, the device or sensor may be capable of measuring more thanone secondary property with sufficient accuracy.

The method 100 includes a step 150 of comparing the calculated value ofthe secondary property with the corresponding measured value of thatsecondary property. If the calculated value differs from the measuredvalue by at least a threshold value, the difference is attributed to theRaman-invisible gas in the sample. Note the specific value of thethreshold will depend on the particular secondary property chosen forthe method 100.

The calculated difference in step 150 may be a positive or negativevalue depending on the choice of the secondary property. With certainsecondary properties, it is expected the measured value of that propertywill be greater than the calculated value if a Raman-invisible componentis present in the sample. But with other secondary properties, it mightbe expected the measured value be less than the calculated value if aRaman-invisible component is present in the sample.

When the difference calculated in step 150 exceeds the threshold value,the method 100 may include a step 160 of calculating the relative amountof the Raman-invisible gas present in the sample. This calculation isperformed using the difference between the calculated value and measuredvalue of secondary property.

The method 100 includes a step 170 of adding the amount of theRaman-invisible component that was calculated in step 160 to therelative composition matrix and re-calculating the amounts of the othercomponents that were already in the relative composition matrix. Aspreviously noted, the amount of each component of the gas sample isexpressed as a percentage of the whole sample. Therefore, adding theRaman-invisible gas to the relative composition matrix requires there-calculation of all the components which were already in the relativecomposition matrix.

In an embodiment of method 100, the method steps may be repeated using adifferent secondary property of the gas mixture. The amount of theRaman-invisible gas determined in the second iteration can be combinedwith the amount of the Raman-invisible gas determined in the firstiteration for a more accurate final value of the amount of theRaman-invisible gas present in the gas sample. The values from eachiteration can be combined through a weighted average, for example. Notethe method is not limited to being run only twice on the gas sample, butmay be performed several times using different secondary properties eachtime and re-calculating the relative composition matrix accordingly eachtime.

The foregoing embodiment of method 100 is not limited to using Ramanspectroscopy on a gas sample. In an alternate embodiment of method 100,an absorption spectroscopy (e.g., infrared or near infrared) may beperformed on the gas sample. In such an embodiment, the remaining stepsof the method are essentially the same. In yet another alternateembodiment of method 100, the substance being analyzed using eitherRaman or absorption spectroscopy is in the condensed phase. Suchanalysis of a solid or liquid phase substance may be for determiningpercent salt content, for example.

A method 200 according to at least one embodiment of the presentdisclosure is shown in FIG. 2 . The method 200 may be performed on amulti-component gas sample contained in a sample chamber.

The method 200 includes a Raman spectroscopic analysis of the gas sampleto generate and to capture a Raman spectrum of the gas sample, which isnot shown in FIG. 2 .

The method 200 includes a step 220 of calculating the relativecomposition of the gas sample from the Raman spectrum and producing amatrix of the components of the gas sample. The step 220 includesnormalizing the components of the matrix by expressing eachRaman-detected component as a ratio of the amount of that component tothe sum of the amounts of all Raman-detected components. That is, eachcomponent is expressed as a percentage of the whole sample. The resultof step 220 is a relative composition matrix containing the amounts ofeach Raman-detected component in the gas sample.

The method 200 includes a step 240 of measuring one or more secondaryproperties of the gas mixture. In the embodiment shown in FIG. 2 , thedensity, pressure, and temperature of the gas sample are measured instep 240, but other secondary properties may be measured and used aswell.

The method 200 includes a step 260 of calculating the amount of theRaman-invisible gas present using formulae derived from the ideal gaslaw and using the measured values of one or more secondary properties ofthe gas. In step 260 the multi-component gas mixture is treated as asimple binary mixture of gases A and B. In this step, gas A of thebinary mixture is the Raman-invisible gas. Gas B of the binary mixtureis the aggregation of the components determined by the Ramanspectroscopy.

To accomplish this simplification and aggregate all the known gases inthe gas sample into one representative gas, gas B, a data set of knowngases is provided. The substance and amount of each constituent must beaccurately determined to enable an accurate calculation of secondaryproperties of the representative gas. Without the accuratecharacterization of the gas sample by the Raman spectroscopy, forexample, there is no reasonable expectation of accuracy in treating thegas mixture as a binary mixture.

Note though the description of method 200 specifies a Ramanspectroscopic analysis of the sample, other spectroscopy methods, suchas NIR or IR absorption spectroscopy, may be employed as well. Forexample, the concentration of homonuclear diatomics such as O₂, N₂,etc., which are normally invisible to IR absorption spectroscopy, may bedetermined using the analysis methods of this disclosure.

Step 260 includes a step 262 of calculating the average molar mass ofgas B of the binary gas mixture (i.e., MW_(B) as shown in FIG. 2 ). InFIG. 2 , the molarity of each Raman-detected component is represented byX_(i). The molar mass of each Raman-detected component is represented byMW_(i). The number of Raman-detected components is N.

Step 260 includes a step 264 of calculating the inverse of thecompressibility factor of the gas sample from the values of the density,temperature, and pressure of the gas sample that were measured in step240.

Step 260 includes a step 266 in which the molar fraction of gas A (i.e.,X_(A) as shown in FIG. 2 ), the Raman-invisible gas, is calculated. Theinputs to the calculation of the molar fraction of gas A include theinverse compressibility factor calculated in step 264, the average molarmass of gas B calculated in step 262, and the average molar mass of gasA. The average molar mass of gas A is determined by specifying thesubstance of gas A. That is, the substance of gas A must be known andspecified so that an appropriate value of the average molar mass of gasA can be used to calculate the molar fraction of gas A.

The method 200 includes a step 270 of adding the amount of theRaman-invisible component that was calculated in step 266 to therelative composition matrix and re-calculating the amounts of the othercomponents that were already in the relative composition matrix. Aspreviously noted, the amount of each component of the gas sample isexpressed as a percentage of the whole sample. Therefore, adding theRaman-invisible gas to the relative composition matrix requires there-calculation of all the components which were already in the matrix.

A method 300 according to at least one embodiment of the presentdisclosure is shown in FIG. 3 . The method 300 may be performed on amulti-component gas sample contained in a sample chamber.

The method 300 includes a Raman spectroscopic analysis of the gas sampleto generate and to capture a Raman spectrum of the gas sample, which isnot shown in FIG. 3 .

The method 300 includes a step 320 of calculating the relativecomposition of the gas sample from the Raman spectrum and producing amatrix of the components of the gas sample. The step 320 includesnormalizing the components of the matrix by expressing eachRaman-detected component as a ratio of the amount of that component tothe sum of the amounts of all Raman-detected components. The result ofstep 320 is a relative composition matrix containing the amounts of eachRaman-detected component in the gas sample.

The method 300 includes a step 330 of making an initial estimate of themolar amount of the Raman-invisible component. The initial estimate canbe based on an expectation of the amount of the Raman-invisiblecomponent present, or the initial estimate can be based on prioranalyses of similar gas mixtures. The value of the initial estimate isnormalized; that is, it is expressed as a percent of the whole sample.

The method 300 includes a step 340 of measuring one or more secondaryproperties of the gas mixture. In the embodiment shown in FIG. 3 , thedensity, pressure, and temperature of the gas sample are measured instep 340, but other secondary properties may be measured and used aswell.

The method 300 includes a step 350 of calculating the compressibilityfactor of the gas mixture using the secondary properties measured instep 340.

The method 300 includes a step 360 of calculating the normalized molarvalue of the Raman-invisible component. Step 360 may be performed one ormore times to calculate iteratively a final value for theRaman-invisible component. The first iteration of step 360 uses theinitial estimate of the molar value of the Raman-invisible componentfrom step 330. A subsequent iteration of step 360 uses the molar valueof the Raman-invisible component calculated in the iteration of step 360previous to that subsequent iteration.

The method step 360 includes a step 362 of re-calculating the relativecomposition matrix of the gas sample. In step 362 the molar value of theRaman-invisible gas is added to the relative composition matrix. Or ifthe molar value of the Raman-invisible gas is already in the matrix froma previous iteration of method step 360, that value is replaced with thenew value described in the preceding paragraph. The molar fractions ofthe other components in the matrix are re-calculated to account for theadded or new molar value of the Raman-invisible gas.

The method step 360 includes a step 364 of calculating thecompressibility factor of the gas mixture using the complete compositionmatrix as an input.

The method step 360 includes a step 366 of comparing the value of thecompressibility factor calculated in step 364 with the value of thecompressibility factor calculated in step 350. If the difference betweenthe two calculated values of the compressibility factor is less than athreshold, the iterations of step 360 are complete. The last estimatedmolar value of the Raman-invisible component is the final value.

However, if the difference calculated in step 366 is greater than thethreshold, the method step 360 may include a step 368 of adjusting theestimated molar value of the Raman-invisible component to reduce thedifference in the two calculated compressibility factors in the nextiteration of method step 360. The method step 360 may be executediteratively to reduce the difference between the two calculatedcompressibility factors to a value less than the threshold.

When the iterations of step 360 have been completed, i.e., when thedifference between the two calculated values of the compressibilityfactor is less than the threshold, the complete mixture compositionmatrix as determined in step 362 of the last iteration of method step360 is output as the mixture composition matrix.

While various embodiments of a method for analyzing the components of agas mixture have been described in considerable detail herein, theembodiments are merely offered by way of non-limiting examples of thedisclosure described herein. It will therefore be understood thatvarious changes and modifications may be made, and equivalents may besubstituted for elements and steps thereof, without departing from thescope of the disclosure. Indeed, this disclosure is not intended to beexhaustive or to limit the scope of the disclosure.

Further, in describing representative embodiments, the disclosure mayhave presented a method and/or process as a particular sequence ofsteps. However, to the extent that the method or process does not relyon the particular order of steps set forth herein, the method or processshould not be limited to the particular sequence of steps described.Other sequences of steps may be possible. Such sequences may be variedand still remain within the scope of the present disclosure. Therefore,the particular order of the steps disclosed herein should not beconstrued as limitations of the present disclosure.

What is claimed is:
 1. A method for analyzing the composition of amulti-component gas sample, comprising: determining a first compositionof a multi-component gas sample using a spectroscopic device;calculating a relative composition matrix containing a normalized molaramount of each component of the multi-component gas sample, based on thefirst composition; calculating an average molar mass of the relativecomposition matrix using a molar mass and the normalized molar amount ofeach component of the relative composition matrix; measuring atemperature, a pressure, and a density of the gas sample and calculatingan inverse compressibility factor of the gas sample therefrom;calculating a molar amount of a spectroscopic-invisible gas using theaverage molar mass, the inverse compressibility factor, and a molar massof the spectroscopic-invisible gas; adding the molar amount of thespectroscopic-invisible gas to the relative composition matrix; andadjusting the normalized molar amount of each component in the relativecomposition matrix to account for the molar amount of thespectroscopic-invisible gas.
 2. The method of claim 1, wherein thespectroscopic device is a Raman spectroscopic device andspectroscopic-invisible is defined as invisible to Raman spectroscopy.3. The method of claim 1, wherein the spectroscopic device is a nearinfrared/infrared absorption spectroscopic device andspectroscopic-invisible is defined as invisible to nearinfrared/infrared absorption spectroscopy.
 4. A method for analyzing thecomposition of a multi-component gas sample, comprising: determining afirst composition of a multi-component gas sample using a spectroscopicdevice; calculating a relative composition matrix containing anormalized molar amount of each component of the multi-component gassample, based on the first composition; measuring a temperature, apressure, and a density of the gas sample and calculating a firstcompressibility factor of the gas sample therefrom; estimating a molaramount of a spectroscopic-invisible gas; adding the estimated molaramount of the spectroscopic-invisible gas to the relative compositionmatrix; adjusting the normalized molar amount of the other components inthe relative composition matrix to account for the estimated molaramount of the spectroscopic-invisible gas; calculating a secondcompressibility factor of the gas sample using the relative compositionmatrix; determining whether a difference between the firstcompressibility factor and the second compressibility factor exceeds athreshold, wherein upon determining that the difference exceeds thethreshold: adjusting the estimated molar amount of thespectroscopic-invisible gas in the relative composition matrix such thatthe difference between the first compressibility factor and the secondcompressibility factor is reduced; repeating the adjusting of thenormalized molar amount of each component in the relative compositionmatrix, the calculating a second compressibility factor, and thedetermining whether a difference between the first compressibilityfactor and the second compressibility factor exceeds the threshold untilthe difference does not exceed the threshold.
 5. The method of claim 4,wherein the spectroscopic device is a Raman spectroscopic device andspectroscopic-invisible is defined as invisible to Raman spectroscopy.6. The method of claim 4, wherein the spectroscopic device is a nearinfrared/infrared absorption spectroscopic device andspectroscopic-invisible is defined as invisible to nearinfrared/infrared absorption spectroscopy.